Oct 12, 2006

Fossilized embryos predating the Cambrian Explosion by 10 million years provide evidence that early animals had already begun to adopt some of the structures and processes seen in today's embryos, say researchers from Indiana University Bloomington and nine other institutions in this week's Science. James Hagadorn of Amherst College led the multi-disciplinary international collaborative project.

The researchers from the U.S., U.K., China, Sweden, Switzerland and Australia report the first direct evidence that primitive animals 550 million years ago were capable of asynchronous cell division during embryonic development. Asynchronous cell division allows the formation of unique shapes.

"We're learning something about how the very earliest multicellular animals formed embryos and how the embryos developed," said IU Bloomington biologist Rudolf Raff, a coauthor of the report. "This gives us an enormous and entirely surprising look at half-a-billion-year-old embryos in the act of cleaving. What a window on the past. We've had no prior idea what they might have done."

The researchers also believe they've identified specialized structures inside the cells, such as bubble-like vesicles that the cells might have used to transport, store or metabolize molecules. Slight aberrations during the fossilization of dead embryonic cells even reveal what appear to be dividing nuclei. It was assumed such structures existed in early animals, but until now, no known fossils of the structures existed.

The scientists procured 162 "relatively pristine" animal embryo fossils from the Doushantuo Formation of south central China. The embryos were still encased in a fertilization envelope, a protective husk that likely aided the preservation of the embryos long enough for fossilization to occur. To inspect the fossils' surfaces and even innards, the scientists used a bevy of imaging techniques, including X-ray computed tomography, scanning electron microscopy, transmission electron microscopy and thin-section petrography.

Even in larger embryo fossils estimated to contain 1,000 cells or more, the scientists did not observe a blastocoel, a fluid-filled gap in the middle of the embryo and a common feature among modern animal embryos. Raff said there are two likely explanations for the observation: "Either these embryos are primitive and don't have a clear blastocoel, or a blastocoel existed but didn't survive the preservation process."

In another study of embryos published by Raff, IUB Department of Biology Chair Elizabeth Raff and colleagues earlier this year, the scientists reported blastocoels were not always preserved under the kinds of preservation conditions that may have been involved in the formation of fossil embryos.

The Raffs and research associate F. Rudolf Turner provided electron micrographs of internal structures such as embryonic lipid vesicles in modern embryos that served as the key comparisons with structures observed in the fossil embryos, and were a source of expertise on the asynchronous cleavage of embryonic cells. Living embryos contain cell structures that form the basis for interpreting structures seen in fossils.

Biologists can provide critical information from living embryos to the studies of the fossils. Until only recently, many paleontologists doubted claims that fossilized embryos hundreds of millions of years old could exist. The Raffs and Philip Donoghue were lead co-authors of a paper in the April 11, 2006, Proceedings of the National Academy of Sciences demonstrating the feasibility of the fossilization of embryos. The present analysis of fossilized embryos in Science leaves even less room for doubt that such finds are real.

Fossilized embryos are very rare. Intact fossil embryos are even rarer. The Doushantuo Formation has proved a boon to paleontologists and evolutionary developmental biologists interested in the evolution of animal species during and prior to the Cambrian Explosion, a dramatic time period in which animals became bigger, more diverse, ecologically dominant and, in the late Stephen J. Gould's opinion, a lot more wonderful.

Related Stories

The first detailed study of the otoliths of extant African annual killifish species provides vital information that will facilitate the identification of fossil forms of this group and contribute to the elucidation ...

If you never understood what "ontogeny recapitulates phylogeny" meant in high school, don't worry: biologists no longer think that an animal's "ontogeny", that is, its embryonic development, replays its entire ...

A group of researchers from the University of Helsinki and the Universitat Autònoma de Barcelona have been able experimentally to reproduce in mice morphological changes which have taken millions of years to occur. Through ...

Regeneration is one of the most tantalizing areas of biological research. How are some animals able to regrow body parts following injury? Why can't humans do the same thing? Can scientists learn the secrets ...

During evolutionary diversification of vertebrate limbs, the number of toes in even-toed ungulates such as cattle and pigs was reduced and transformed into paired hooves. Scientists at the University of Basel have identified ...

Recommended for you

Certain specimens of the fossil Dickinsonia are incomplete because ancient currents lifted them from the sea floor, a team of researchers led by paleontologists at the University of California, Riverside has fo ...

For the earliest Egyptologists, a trip to the Egyptian Museum in Turin was considered indispensable. The museum's new director is seeking to return the almost 200-year-old museum to its one-time prominence, ...

Working with an international team, paleontologists at the University of Zurich have discovered two new species of Saurichthys. The ~242 million year old predatory fishes were found in the fossil Lagerstätte ...

In the 1990s the discovery of the oldest man made and completely preserved wooden hunting weapons made the Paleolithic excavation site in Schoningen internationally renowned. Contained within the 300,000 ...

Asier Gómez-Olivencia, an Ikerbasque researcher at the UPV/EHU, has led a piece of research that has produced a 3D reconstruction of the remains of a two-year-old Neanderthal recovered from an excavation ...

User comments : 0

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.